The present invention is directed to mass flow sensors of the thermal type, in general, and more specifically to a mass flow sensor including a heated probe having a predetermined length disposed in the path of mass flow; at least two first temperature measuring devices disposed at different points along said length for measuring temperatures at said points of the heated probe in the presence of the mass flow, a second temperature measuring device for measuring the temperature of the mass flow, and means for determining mass flow as a function of said temperature measurements of the heated probe and said temperature measurement of the mass flow, and a method of measuring mass flow.
Mass flow sensors of the thermal type generally involve exposing a self-heated resistance element to the media flow and measuring the cooling effect of the media on the self-heated element, conventionally by its resistance change. If the media temperature can vary, a temperature sensor measurement is also made to get an accurate ambient temperature measurement. A resistance measurement of the self-heated element can be related to media mass flow. This method is also called hot-wire (or hot-film) anemometry and has been used since about 1950.
A similar approach involves locating a heater in the path of the media or around the pipe or duct through which the media is flowing with temperature sensors disposed on either side. For measuring the temperature T1 of the media upstream of the heater and the temperature T2 downstream of the heater. T1 is used for reference purposes while T2 located downstream of the heater is used to sense the additional heat transferred from the heater by the moving fluid. The heat input (QH) and the mass flow rate (QM) are related by the equation:
QH=c(T2xe2x88x92T1)QM,
where c is the specific heat of the fluid.
If the heater is located directly in the media flow, high power consumption is often required. An alternate approach is the xe2x80x9cboundary-layerxe2x80x9d flowmeter in which only the layer of the media fluid closest to the pipe or duct wall if heated and subjected to the aforementioned temperature measurements.
There are a number of variations of the thermal approach being used today including those using direct heating of the media fluid, and a single or dual probes containing self-heated temperature sensing elements. The method using direct heating of the fluid does so in a small xe2x80x9cbypassxe2x80x9d duct and is referred to as a xe2x80x9cparallel methodxe2x80x9d of measurement. A primary alternative is the xe2x80x9cseries methodxe2x80x9d which places a probe or probes directly in the media flow path within the pipe or duct. Also, the self-heated and sensor elements use a variety of technologies such as platinum, tungsten or other resistance temperature devices (RTDs), thermistor deposited films, and the like, for example.
Mass flowmeters of the thermal type are most often used in aircraft applications because the sensors are small. Light weight, easy to install, flexible for different ranges, and create minimal restrictions to flow in the piping or duct, which results in a low pressure drop across the measurement section thereof. Such applications include engine bleed air, environmental control systems for avionics cooling, cabin environmental air, fuel cell gas flow and the like, for example.
One such flow sensor uses a thermal based immersion sensor incorporating a single probe which contains an ambient element and a self-heated element. The ambient element senses the ambient temperature of the media flow in the pipe or duct while the self-heated element measures the cooling effect of the media flow in the duct. By monitoring the cooling effect of the self-heated element and the ambient temperature of the duct media flow, the mass flow can then be determined by a signal processing unit. The signal processor receives the cooling effect and ambient temperature measurements from each sensor and calculates the corresponding mass flow rate. Typically, a cooling effect sensor is calibrated for a low flow value in order to trigger a means of increasing the cooling flow, and, once cooling is achieved, reduces cooling much like a thermostat controls temperature, for example. The principles of the sensor/signal processor are relatively simple. Two basic parameters (cooling effect and ambient temperature) are measured and the signal processor mathematically combines these measurements to provide the desired output. A drawback for this type of sensor is incomplete isolation of the self-heated element from the ambient element. Conductive or convective heat transfer between the probes will result in an error in measurement. Also, the effect of the mounting surface temperature can cause errors in measurement, especially if the probe is short.
Another thermal type of mass flow sensor uses two platinum resistance temperature (PRT) elements disposed in the path of media flow to directly measure mass flow. One element is used to sense ambient flow temperature while the other is self-heated to sense flow velocity. The elements may be contained in the same probe or two different probes. By self-heating the sensing element to a fixed temperature difference higher than ambient, the amount of power supplied to the element will be indicative of mass flow. Processing electronics include a xe2x80x9cflow bridgexe2x80x9d which is used to power the self-heated element and convert the changes in resistance of the PRT elements to a signal which is representative of mass flow. It is recognized that the resistance of the sensing elements of these type sensors is indicative of temperature only if several error sources are identified and minimized through sophisticated electronics.
One such source of error is the heat generated in the ambient sensor resistance wire due to passage of the current utilized to measure the resistance. This heating effect (I2 R or Joule heating) results in a higher element temperature than the ambient and causes an erroneous reading, especially at low flow rates. This error is referred to as the self-heating error. The self-heating error, defined as the temperature difference between the surrounding (ambient) temperature and the temperature at which the sensing element equilibrates, is a function of the element resistance (R), the current flowing through the element (I) and the heat transfer field around the sensor.
Self heating of the cooling effect sensor also causes an error in measurement. Since the RTD is a non-linear device, there is typically an error associated with ambient temperature compensation. Another problem associated with these immersion type devices is the reduced sensitivity of the device at higher mass flows. It is a phenomena that occurs as a natural result of the relative reduced heat transfer of a probe in crossflow. Sometimes this reduction is enhanced by the slope of the RTD which can limit the application of this type of device.
Another similar thermal type mass flow sensor which is based on the velocity effect and temperature measurements utilize thermister sensors for measuring the dissipated heat and ambient temperatures. One such sensor uses a single probe which contains dual thermisters at the tip for measuring the flow of dissipated heat of heater windings wrapped around their housing. The dual thermisters essentially measure the temperature of the heater. There are also dual thermisters, located in open flow, for measuring the ambient temperature. A controller provides the heater windings with a constant current or a temperature driven voltage. By using dual thermisters to measure ambient temperature as a reference, the tip thermisters may be used to infer flow velocity of the media across the heater windings. While thermisters are less costly then PRTs, they are limited in range of temperature covered as compared to PRTs and need rather complex signal conditioning circuits. In addition, since the thermisters are not self-heated, these thermister-based sensors require an additional heater element for heating the probe.
The present invention includes an improved sensor design for and method of measuring mass flow, which overcomes the aforementioned drawbacks of the current mass flow sensors.
In accordance with one aspect of the present invention, a mass flow sensor comprises: a probe having a predetermined length for disposition in the path of mass flow; means for heating the probe; at least two first temperature measuring devices disposed at different points along the length for measuring temperatures of the heated probe at such points in the presence of the mass flow; a second temperature measuring device for disposition in the path of mass flow for measuring the temperature of the mass flow; and means for determining mass flow as a function of the temperature measurements of the first and second temperature measuring devices.
In accordance with another aspect of the present invention, a method of measuring mass flow comprises the steps of: disposing a probe of a predetermined length in the path of mass flow; heating the probe; measuring the heated probe temperature at at least two different points along its length in the presence of the mass flow; measuring the temperature of the mass flow; and determining mass flow as a function of the temperature measurements of the heated probe and mass flow.
In accordance with yet another aspect of the present invention, a mass flow sensor comprises: at least one probe having a predetermined length for disposition in the path of mass flow; means for heating the at least one probe; at least three temperature measuring devices disposed in proximity to the at least one probe for generating signals representative of the temperature of the heated probe along its length and the temperature of the mass flow; means for generating at least two signals representative of temperature differentials in the vicinity of the at least one probe from the signals generated by the at least three temperature measuring devices; and means for processing at least one of the temperature differential signals to generate a signal representative of mass flow.